1. Field of the Invention
This invention relates generally to magnetic recording hard disk drives, and more particularly to a disk drive with a dual-stage actuator for positioning the read/write heads.
2. Description of the Related Art
Magnetic recording hard disk drives with dual-stage actuators for positioning the read/write heads on the disks have been proposed. A rotary voice-coil-motor (VCM) is typically the primary actuator, with the secondary actuator attached to the VCM and the read/write heads attached to the secondary actuator. A servo control system receives servo positioning information read by the read/write heads from the data tracks and generates control signals to the primary and secondary actuators to maintain the heads on track and move them to the desired tracks for reading and writing of data. As in conventional single-stage actuator disk drives, each read/write head is attached to the end of a head carrier or air-bearing slider that rides on a cushion or bearing of air above the rotating disk. The slider is attached to a relatively flexible suspension that permits the slider to “pitch” and “roll” on the air bearing, with the suspension being attached to the end of the VCM actuator arm. The secondary actuator is typically a piezoelectric or electrostatic milliactuator or microactuator located on the VCM actuator arm for driving the suspension, or on the suspension between the suspension and the slider for driving the slider, or on the slider for driving just the read/write head.
The conventional servo control system for a typical dual-stage actuator disk drive uses a controller designed to assure stability of the VCM with adequate stability margins as if it were to operate without the secondary actuator. Then the controller for the secondary actuator is designed to achieve the desired combined dual-stage bandwidth. The secondary actuator control loop and the combined dual-stage control loop are also designed to ensure adequate stability separately and jointly with the other control loops. This type of servo control system is satisfactory for limited increases in the bandwidth above what is achievable with only the VCM.
In co-pending application Ser. No. 10/802,601 filed Mar. 16, 2004, titled “MAGNETIC RECORDING DISK DRIVE WITH DUAL-STAGE ACTUATOR AND CONTROL SYSTEM WITH MULTIPLE CONTROLLERS”, and assigned to the same assignee as this application, a dual-stage actuator disk drive is described that operates with an improved servo control system that has two controllers. One controller is a dual-stage controller that simultaneously generates a primary actuator control signal and a secondary actuator control signal, and uses a degraded-stability primary actuator controller design with relatively high low-frequency open-loop gain and a secondary actuator controller design that provides stability and high mid-frequency to high-frequency open-loop gain resulting in increased bandwidth. The other controller is a single-stage controller that generates only a primary actuator control signal and uses a stable VCM-only controller design. If a potential failure of the secondary actuator is detected, the servo control system selects the single-stage controller.
In dual-stage actuator disk drives with either the conventional servo control system or the control system of the co-pending application, a failure of the secondary actuator will result in reduced performance and may lead to loss of data and/or failure of the disk drive.
What is needed is a dual-stage actuator disk drive that can detect actual failure of the secondary actuator and modify the controller parameters if the failure is recoverable.